1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an image forming apparatus and an image forming method that carry out an image forming process in which an image is formed on an image bearing member.
2. Description of the Related Art
In image forming apparatuses, printing is performed in such a manner that a full-color image is formed by sequentially superposing toner images of respective colors on an intermediate transfer belt that is an image bearing member, and the formed full-color image is transferred onto a recording sheet.
Ordinarily, in such image forming apparatuses, a phase mark (registration mark) is formed by toner (developing agent) on the intermediate transfer belt, and toner images of respective colors are superposed on the intermediate transfer belt using the registration mark as the reference position (see Japanese Laid-Open Patent Publication (Kokai) No. H11-295941, for example). By the use of such a registration mark, toner images of respective colors can be registered in forming a full-color image. Also, some image forming apparatuses of the type that uses no registration mark, circumferential length information indicative of the circumferential length of the intermediate transfer is acquired, and the reference position at which image formation is started is determined based on the acquired circumferential length information.
The above-mentioned registration mark can also be used as the reference position for starting the formation of a reference density pattern (patch image) for use in correcting density characteristic information such as the density and tone of a toner image to be formed. It should be noted that how to correct density characteristic information (color calibration) is described in Japanese Laid-Open Patent Publication (Kokai) Nos. 2002-211083 and H07-036230, for example.
In the above-described image forming apparatuses, however, it is necessary to cause the endless intermediate transfer belt to rotate idle until the reference position is identified (detected) on the intermediate transfer belt. Thus, there is downtime (wasted time) corresponding to the time required to identify the reference position at which image formation is (i.e. the time during which the intermediate transfer belt is caused to rotate idle). For this reason, it is impossible to carry out image formation and correction of density characteristic information in an efficient manner.
The surface of the intermediate transfer belt tends to be scratched by rotation of the intermediate transfer belt itself and abutment of a member that abuts on and separates from the intermediate transfer belt. Also, thickness changes and expansion/contraction of the intermediate transfer belt may occur due to long-term usage. For this reason, the physical properties of the intermediate transfer belt change, causing the circumferential length of the intermediate transfer belt to change. This makes it difficult not only to identify the reference position at which image formation is started but also to register images of respective colors constituting a full-color image. In this case as well, there is downtime, and hence image formation cannot be carried out in an efficient manner.